Transport properties and performance of CdZnTe strip detectors

Tousignant, O.; Hamel, L. A.; Courville, J.F.; Macri, J.; Mayer, M.; McConnell, Mark L.; Ryan, J. M.

doi:10.1109/23.682418

Cited by 13 publications

(4 citation statements)

References 16 publications

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“…The analysis of induced photocurrents in pixel detectors (where one electrode has been pixelled into many charge collecting electrodes for imaging or position sensing purposes) and the calculation of the collected charge in different pixels are current topics of interest in the detector community and have been discussed in the literature (e.g. Luke 1995, Hamel and Paquet 1996, Tousignant et al 1998. Our goal is to derive the intrinsic sensitivity of the photoconductor material operating under a constant field in terms of charge transport limitations.…”

Section: Schubweg Limited X-ray Sensitivitymentioning

confidence: 99%

X-ray sensitivity of photoconductors: application to stabilized a-Se

Kasap

2000

J. Phys. D: Appl. Phys.

379

342

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The x-ray sensitivity of a high-resistivity photoconductor sandwiched between two parallel plate electrodes and operating under a constant field is analysed by considering charge carrier generation that follows the x-ray photon absorption profile and taking into account both electron and hole trapping phenomena but neglecting recombination, bulk space charge and diffusion effects. The amount of collected charge in the external circuit due to distributed generation of electrons and holes through the detector is calculated by integrating the Hecht collection efficiency with Ramo's theorem across the sample thickness. The results of the model allow the x-ray sensitivity to be calculated as a function of the applied field, detector thickness and electron and hole ranges (µτ), given the field and energy dependence of the electron and hole pair creation energy, W±, and the energy spectrum of incident x-ray radiation. The sensitivity model was applied to stabilized a-Se that is currently used as a successful x-ray photoconductor in the recently developed flat panel x-ray image detectors. Recent free electron-hole pair creation energy versus electric field data at room temperature and appropriate electron and hole drift mobilities were used to calculate the sensitivity for monoenergetic x-rays at 20 and at 60 keV. For the 20 keV radiation, it was shown that a typical detector thickness of 200 µm (4 × attenuation depth at 20 keV) with currently attainable electron and hole trapping parameters in a-Se was operating optimally, the sensitivity of which can only be increased by further increasing the applied field. With the receiving electrode positively biased, the sensitivity was much more dependent on the hole lifetime than electron lifetime. The absence of hole transport results in a reduction in sensitivity by a factor of about 4.4, whereas the absence of electron transport results in a sensitivity degradation of only 22%. The ratio of hole trapping limited sensitivity to electron trapping limited sensitivity is about 0.3. For a detector of thickness 200 µm operating at 10 V µm-1, the maximum sensitivity is about 220 pC cm-2 mR-1, and this sensitivity degrades by more than 10% when either the electron lifetime falls below ~20 µs or the hole lifetime falls below ~5 µs. When the hole lifetime is very short so that the sensitivity is substantially reduced, the sensitivity versus thickness dependence at a given field exhibits a maximum (an optimal thickness) that is less than that for full absorption. In the case of 60 keV x-ray photons, it is more useful to examine the sensitivity as a function of detector thickness given the practical bias voltage limit. The sensitivity versus thickness behaviour for a given bias voltage exhibits a maximum, that is an optimal thickness, that is less than that for nearly full absorption. Electron lifetimes longer than ~200 µs and hole lifetimes longer than ~10 µs do not significantly affect the sensitivity.

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Section: Schubweg Limited X-ray Sensitivitymentioning

confidence: 99%

X-ray sensitivity of photoconductors: application to stabilized a-Se

Kasap

2000

J. Phys. D: Appl. Phys.

379

342

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“…However, we could guess a non-uniform electric field near the electrode from the nonlinear rise time. Since detrapping occurs in the CZT material [11], it was recognized that limitations would be encountered in applying the Hecht's equation. Also, it is possible that the induced charge Q was not propotional to the pulse height because of the leakage current.…”

Section: Resultsmentioning

confidence: 99%

Measurement of the drift mobilities and the mobility-lifetime products of charge carriers in a CdZnTe crystal by using a transient pulse technique

Cho¹,

Lee²,

Kwon³

et al. 2011

J. Inst.

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In this work we present results on the measurement of the drift mobility and the mobility-lifetime product of charge carriers in a 16-pixellated CdZnTe detector. For the determination of an interaction position based on the pulse rise-time method in a CZT detector, it is necessary to characterize the transport properties governed by drift mobility and lifetime for electrons and holes. In order to extract the transport properties of an electron and a hole, we bombarded 5.5-MeV alpha particles from a 241 Am source and 81-keV gamma rays emitted from a 133 Ba source on the negatively biased contact of the CZT detector. A time-of-flight (TOF) method was used to measure the electron drift mobility at room temperature whose value turned out to be 906.4 cm 2 /V• s. With the Hecht's equation, the electron mobility-lifetime product was also determined from the bias-dependent alpha response and was equal to (9.88 ± 2.33) × 10 −3 cm 2 /V. On the other hand, the hole mobility-lifetime product was evaluated by a model based on the average charge collection efficiency which accounts for the absorption probability with a given photon energy. By using a single parameter fitting of the model, we obtained the hole mobility-lifetime product of (8.28 ± 0.17) × 10 −4 cm 2 /V. KEYWORDS: Charge transport and multiplication in solid media; Gamma detectors; Instrumentation and methods for time-of-flight (TOF) spectroscopy

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“…An early attempt to create computational models of CdZnTe strip detectors was made by Herman et al [7] and Tousignant et al [8] using analytical solutions of the continuity equations and Monte Carlo simulation. Although the results obtained were qualitatively good there was a lack of proper modelling of the response of various parameters, for example the signal readout [7] or the lack of three-dimensional information [8]. Later work by Mathy et al [9] used the finite element method to obtain the solution to the continuity equation in pixelated CdZnTe, the main focus of this work being the biparametric spectra analysis.…”

Section: Introductionmentioning

confidence: 99%

Computational modelling of pixelated CdZnTe detectors for x- and γ- ray imaging applications

2012

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Cadmium Zinc Telluride (CdZnTe) detectors are currently used in medical imaging systems employing γ-ray photons. As new imaging techniques such as photon-counting and energy-weighted x-ray imaging are gaining research interest, CdZnTe is seen under a new light for potential use in computed tomography, tomosynthesis and other x-ray imaging applications. However, being relatively expensive, CdZnTe could be favoured by advanced computational modelling to assist in detector and imaging system optimisation. In this work, pixelated CdZnTe detectors are computationally modelled using an integrated framework that combines the Finite Element and Monte Carlo numerical methods to obtain realistic detector models.Various detector thickness and pixel sizes are designed and their performance is investigated in terms of charge induction efficiency, detection efficiency and energy resolution. Detection efficiency and energy resolution are assessed for monoenegergetic photon beams within the energy range used in medical x-ray imaging applications such as mammography and computed tomography. Some of the capabilities of the framework are demonstrated. Small pixel sizes, below 100µm are prone to charge transport effects such as diffusion, especially in larger thickness (> 0.5 mm) and may have limited use in pixelated geometries. Detection efficiency is affected by fluorescence and photon escape as thickness and pixel size decrease. Energy resolution is affected by beam geometry and can vary from ∼3% to 11% depending on the beam width. The framework provides a generic platform and a powerful tool that can be used in the design and optimisation of semiconductor detectors made from any semiconductor material, imaging systems and signal correction techniques. KEYWORDS: Solid state detectors; X-ray detectors; Detector modelling and simulations II (electric fields, charge transport, multiplication and induction, pulse formation, electron emission, etc); Detector modelling and simulations I (interaction of radiation with matter, interaction of photons with matter, interaction of hadrons with matter, etc)

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Transport properties and performance of CdZnTe strip detectors

Cited by 13 publications

References 16 publications

X-ray sensitivity of photoconductors: application to stabilized a-Se

X-ray sensitivity of photoconductors: application to stabilized a-Se

Measurement of the drift mobilities and the mobility-lifetime products of charge carriers in a CdZnTe crystal by using a transient pulse technique

Computational modelling of pixelated CdZnTe detectors for x- and γ- ray imaging applications

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